1. Field of the Invention
The present invention relates generally to an optical semiconductor device. More particularly, the invention concerns the optical semiconductor device in which an optical transmitting element and a light receiving element are formed integrally with each other and which is advantageously suited for optical communication through space serving as a transmitting medium.
2. Description of the Prior Art
Owing to remarkable progress in the development of light emitting elements and light receiving elements in recent years, the so-called optical communications in which light is made use of as the information carrier tend to be increasingly and widely employed.
Among the others, in the case of the optical communication in which space is used as the transmitting medium, the light emitting element is activated in accordance with information to be transmitted to thereby emit light which is then radiated into space in a form of a collimated light beam through an optical lens. At the side of a receiving station, the light beam as received is focussed on the light receiving element through a lens to restore the incoming information.
In order to allow the bi-directional optical communication to be realized, a pair of such light transmission and reception channels are of course required. In this connection, it is noted that the lens having a large diameter of the order of 30 cm has to be used for the optical communication over a long distance (e.g. about several kilometers). Consequently, the optical communication system will then be very expensive. Further, complicated and troublesome maintenance and adjustments such as optical-axis-matching or the like are involved. In these respects, there has been a great demand for improvement.
To this end, it has been proposed that the light emitting element and the light receiving element are combined integrally with each other, wherein a single convex lens is used to serve for both the light transmission and the light reception. In this case, the total number of the convex lenses as required for the optical communication is decreased to a half of that required in the optical communication system where the light emitting element and the light receiving element are separately provided at each of the terminals. The maintenance procedures will then be improved.
However, the optical semiconductor device incorporating therein integrally the light emitting or transmitting element and the light receiving element has heretofore been fabricated by forming successively a number of compound semiconductor layers on a GaAs-substrate through epitaxial growth in a laminated multi-layer structure in which a light emitting pn-junction and a light receiving pn-junction are present, as is disclosed in Japanese Laid-Open Patent Application No. 48493/1979, for example. Consequently, manufacture of such integrated optical semiconductor device requires much complicated fabricating processes. Besides, because the light emitting pn-junction and the light receiving pn-junction are formed in a common region in an overlapping relation to each other with a small distance therebetween, the light transmitting or sending operation and the light receiving operation can not be carried out simultaneously, involving great difficulty in attaining a high efficiency in the optical communication.